Fix the replication failure caused by incorect initialization of
THD::invoker_host && THD::invoker_user.

Breakdown of the failure is this:
Query_log_event::host and Query_log_event::user can have their
LEX_STRING's set to length 0, but the actual str member points to
garbage. Code afterwards copies Query_log_event::host and user to
THD::invoker_host and THD::invoker_user.

Calling code for these members expects both members to be initialized.
Eg. the str member be a NULL terminated string and length have
appropriate size.